In the microelectronics industry as well as in other industries involving construction of microscopic structures (e.g., micromachines, magnetoresistive heads, etc.), there is a continued desire to reduce the size of structural features. In the microelectronics industry, the desire is to reduce the size of microelectronic devices and/or to provide greater amount of circuitry for a given chip size.
Effective lithographic techniques are essential to achieve reduction of feature sizes. Lithography impacts the manufacture of microscopic structures not only in terms of directly imaging patterns on the desired substrate, but also in terms of making masks typically used in such imaging. Typical lithographic processes involve formation of a patterned resist layer by patternwise exposing the radiation-sensitive resist to an imaging radiation. The image is subsequently developed by contacting the exposed resist layer with a base solution, typically an aqueous alkaline developer, to selectively remove portions of the resist layer to obtain the desired pattern. The pattern is subsequently transferred to an underlying material by etching the material in openings of the patterned resist layer. After the transfer is complete, the remaining resist layer is removed.
For many lithographic imaging processes, the resolution of the resist image may be limited by anomalous effects associated with refractive index mismatch and undesired reflections of imaging radiation. To address these problems, antireflective coatings are often employed between the resist layer and the substrate (bottom antireflective coating, also known as BARC) and/or between the resist and the atmosphere in the physical path along which the imaging radiation is transmitted (top antireflective coating, also known as TARC).
For immersion lithography, there are some concerns that certain components in the photoresist may leach out to the immersion medium and change the performance of the photoresist, or that the immersion medium may diffuse into the photoresist and affect the acid generation thereby adversely interfering with the chemical amplification mechanism. To alleviate these problems, a topcoat material can be used between the immersion medium and the resist-coated wafer.
In the case of dry lithographic processes, such as dry 193 nm lithography (not involving an immersion fluid in the radiation exposure step), the atmosphere would typically be air. In the case of immersion lithography, the atmosphere would typically be water. Water has a refractive index (n) value of around 1.437 at 193 nm. Thus, if future immersion lithography requires fluids having refractive index (n) values above 1.6, the atmosphere would likely be hydrocarbons.
The performance of an antireflective coating composition is largely dependent on its optical characteristics at the imaging radiation wavelength of interest. A general discussion regarding typically desired optical characteristics of TARCs can be found in U.S. Pat. No. 6,274,295. Among the optical parameters of interest are the refractive index, the reflectance and the optical density of the TARC.
The antireflective coating composition must also have the desired physical and chemical performance characteristics in the context of its use in contact directly with, or in close proximity to, the resist layer and in the context of the overall lithographic process (irradiation, development, pattern transfer, etc.). Thus, the TARC should not excessively interfere with the overall lithographic process. It is highly desirable that a TARC is removed during the image development step which typically involves dissolution of a portion of the resist in an aqueous alkaline developer solution.
The existing commercial TARC compositions do not possess the combination of optical properties and physical and chemical performance characteristics needed for immersion lithography. For example, some prior art TARC compositions have a desired refractive index below 1.6, but are not soluble in aqueous alkaline developers, thereby causing undesired complication and expense of a separate TARC removal step. Other prior art TARC compositions have a desired refractive index, but adversely interact with the resist thus leading to excessive film loss and loss of contrast in the resulting resist image or formation of undesired T-top structures. By “T-top structure”, it is meant that a low solubility thin skin layer forms on top of photoresists to create a “T” shape profile on the resist images. Another prior art TARC compositions containing solvent which is too volatile and flammable, thus is considered unsafe for manufacture.
It has been found that the surface tension of the immersion TARC materials has strong relationship with the prevention of certain defects caused by water droplets left over during high speed exposure scan. A hydrophobic TARC would alleviate this problem by having a high contact angle of water on its surface to prevent meniscus water droplet formation. The hydrophobic surface would also prevent leaching of chemicals to the water. Therefore, there is a need to develop new immersion TARC materials having hydrophobic characteristics.
Thus, there remains a need for TARC compositions suitable for use in immersion lithographic processes that are soluble in aqueous base developers and insoluble in water. It is also desirable that these TARC compositions can be readily prepared from commercially available starting materials, on which water has high contact angle.